High frequency power amplifiers have typically employed vertical power transistors mounted on a high thermal conductivity ceramic substrate such as BeO to electrically isolate the transistor from the mounting flange/heatsink, while providing a low thermal resistance path. There is an inherent trade-off associated with this technique, however, since lower thermal resistance is achieved with a thinner substrate while lower parasitic capacitance dictates the use of thicker substrates.
Transistor amplifiers have been fabricated with vertical transistors mounted directly on the heatsink/ground by using a common drain configuration wherein the drain is coupled to ground. A transistor operated in the common drain configuration, however, exhibits lower gain and f.sub.t than other configurations and results in limited power output and efficiency. In addition, the input voltage swing is limited, compared to other amplifier configurations, due to the nonsymmetrical nature of the device, that is, the gate-source and gate-drain characteristics are generally not the same. Typical prior art configurations having common electrodes (gate and source, respectively) coupled to ground are illustrated in FIGS. 1 and 2. These configurations, however, are not amenable to high efficiency, high power applications at high frequencies.
Furthermore, conventional transistor amplifier circuits having a common grounded electrode introduce negative feedback due to the input and output current pathways having a common segment through which both of these currents flow. For example, input ac current 15 and output ac current 16 in FIG. 1 both flow through a common lead 17 connecting the gate electrode G to ground. As will be shown in the following discussion of the present invention, the novel amplifier disclosed herein establishes separate paths for the input and output currents, thereby preventing negative feedback.
The above discussion regarding the disadvantages of conventional FET amplifiers is also applicable to conventional BJT devices, where the common drain, common gate, and common source configurations are analogous to common collector, common base, and common emmiter configurations, respectively.